Power table saws have been in common use for many years and many accessories have been developed to improve operator safety. Table saws are provided with a blade guard to cover the rotating blade while the saw is in use. However, the guard must be easily removed to facilitate certain special cuts. Often, the operator fails to reinstall the blade guard after the special cut is completed. As a result, many injuries occur each year when saws are operated without the blade guard being properly in place.
Manufacturers have attempted to introduce interlock systems that would detect the presence or absence of the blade guard and allow operation of a saw in the absence of a blade guard only when a special key activation or other interlock override signal has been generated. However, the existence of a dusty, physically rugged environment in the vicinity of the saw blade and the need to accommodate large work pieces have until now rendered such systems unreliable or impractical.
Two examples of blade guard systems are shown in U.S. Pat. No. 3,249,134 issued to Vogl et al. for SAW AND DADO GUARD and U.S. Pat. No. 3,754,493 issued to Niehaus et al. for CIRCULAR SAW BLADE GUARD . The Vogl patent discloses a power saw blade guard that has two independent side panels that adjust relative to the work surface to accommodate the work piece being cut. The entire guard can be easily pivoted away from the blade prior to starting the saw motor.
The Niehaus patent discloses a power saw blade guard that normally rests on the saw table and adjusts to the height of a work piece passing through the saw blade. The Niehaus patent further discloses a guide plate or splitter positioned in line with the saw blade to support the guard and separate a cut work piece after the work piece is passed through the saw blade to prevent the work piece from binding on the blade. When the guide plate is secured in place, the blade guard mounted on the guide plate has a limited swing angle so as to prevent the guard from being tilted back during normal operation. To prevent injuries, this guard, along with its guide plate, must be in place prior to startup and remain in place over the saw blade while the work piece is passing through the saw blade. It is left up to the operator to make sure the guard and the guide plate is in place. The guard and the guide plate can be easily pivoted away, leaving the saw blade exposed while the table saw is running.
Another type of power table saw guard is disclosed in U.S. Pat. No. 5,181,447 to Hewitt. The Hewitt blade guard has a safety power cut-off device that cuts off power to the table saw when the blade guard is pivoted away from the saw blade. This prevents open exposure of the rotating blade to the operator when the guard is tilted away from the blade while the blade is turning. However, this blade guard has several drawbacks. The guard does not self-adjust relative to the working surface and does not stay in contact with the work piece as it is being cut. The guard must be manually adjusted to a fixed height above the table that is sufficient to cut a work piece prior to cutting a work piece. This means that the blade remains exposed to the extent of the height of the work piece. Further once an operator cuts a thick work piece it is likely that a subsequent, thinner work piece will be cut without adjusting the blade guard downward. This will leave more exposure to the blade than necessary during the subsequent cut. In addition, the guard does not adequately enclose the blade so as to protect an operator from flying wood fragments and broken saw teeth. The guard must be mounted on a cantilevered arm that limits the width of a work piece that can be cut. However, the guard does have an interlock circuit and a keylock override that prevents pivoting of the guard completely away from the saw blade unless the keylock override is activated.
The Occupational Safety and Health Administration (OSHA) has established rules specifically for table saws. They require that each circular hand-fed ripsaw be guarded by a hood that completely encloses the portion of the saw above the material being cut. The hood and mounting must allow the hood to automatically adjust itself to the thickness of the material being cut. This same regulation is required for the table saw as well.
There has been a long recognized need to implement a safety interlock circuit for use with this type of self adjusting, continuous contact blade guard. However, the harsh environment in the vicinity of a saw blade has prevented the practical realization of a blade guard electro-mechanical interlock system.
A low cost, highly reliable power tool safety device in accordance with the invention includes a proximity detector detecting the relative proximity between a power cutting tool guard and a table top and an interlock system that prevents startup of the power tool when the guard is not in place. The safety device may be effectively used with self adjusting, continuous contact power table saw blade guards as well as other types of cutting tool guards.
The proximity detector includes at least one permanent magnet permanently secured to the guard to generate a magnetic field and at least one magnetic field intensity detector which may be a normally open magnetic reed switch disposed on a side of the table opposite the working surface at a detection location opposite to the permanent magnet. This placement keeps all of the wiring, electrical contacts and sensors under the table and away from the saw blade, the dust and the constant vibrations of the blade guard. The sensitivity of each reed switch to a magnetic field is selected to cause each reed switch to close when the guard is properly positioned in guarding relationship to the cutting tool. Each magnetic reed switch has an open state and a closed state and is coupled in series between the electric starting circuit and a power source. Each reed switch detects the predetermined relative proximity by remaining in the open state when the predetermined relative proximity is not detected and switching to the closed state when the predetermined relative proximity is detected. The proximity detector generates an electrical proximity signal indicating whether or not the guard is within the predetermined relative proximity to the working surface, the electrical proximity signal being communicated to the interlock system.
The interlock system prevents use of the power tool when the proximity detector does not detect proximity between the guard and the working surface by placing each magnetic reed switch in series with a motor starter circuit. If the guard is not in place the reed switch opens and prevents energization of the motor starter circuit. Once the motor is started the motor will remain energized even if a thick work piece causes a loss of proximity between the guard and the reed switch.
A key operated bypass switch allows the motor to be started one time without the guard in place to accommodate special cuts or unusual work piece shapes that require operation of the power tool with the guard removed. The requirement for a key assures that unguarded operation can be specially supervised by a teacher or supervisor. An indicator light is illuminated to alert an operator or supervisor that the interlock circuit has been bypassed.